With the rise of the environmental protection movement over recent years, restrictions on emissions of carbon dioxide and other exhaust gases that cause warming have been strengthened. Consequently, the automobile industry is engaging actively in development of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like, to replace vehicles that use fossil fuels such as gasoline, diesel oil and natural gas. As the batteries for such EVs and HEVs, nickel-hydrogen secondary batteries or lithium ion secondary batteries are used. In recent years, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries have come to be used in large numbers for this purpose, because they provide a battery that is both lightweight and high capacity.
EVs and HEVs are now required not only to be environment-friendly, but also to have basic performance as vehicles, that is, acceleration performance, gradient-climbing performance, and other high-level driving capabilities. In order to satisfy such requirements, batteries are needed that have not simply an enhanced battery capacity but also high output. The secondary batteries widely used for EVs and HEVs usually are prismatic sealed secondary batteries in which a generation element is housed inside a prismatic outer can, and the internal resistance of such batteries must be reduced to the extent possible, because large current flows in them when high-output discharge is performed. For this reason, various improvements have been undertaken concerning lowering the internal resistance by preventing welding faults between the electrode plate substrates and the collector members in the generation element of the battery.
There exist the methods of mechanical caulking, welding, and so forth, for electrically joining the electrode plate substrates and the collector members so as to effect electrical collection in the generation element. For electrical collection in batteries that are required to have high output, welding is the appropriate method, since it is likely to realize lower resistance and unlikely to deteriorate over time. In lithium ion secondary batteries, aluminum or aluminum alloy is used as the material for the positive electrode plate substrates and collector members, and copper or copper alloy as the material for the negative electrode plate substrates and collector members to realize lower resistance. However, aluminum, aluminum alloy, copper, and copper alloy have the characteristics of low electrical resistance and high thermal conductivity, so that an extremely large amount of energy is required in order to weld them.
The following methods have long been known as methods for welding together the electrode plate substrates and collector members that constitute the generation element:
1) Laser welding
2) Ultrasonic welding
3) Resistance welding
As regards the laser welding method, aluminum, aluminum alloy, copper, and copper alloy, which are the materials to be welded, have a high reflectivity of around 90% with respect to the YAG (yttrium-aluminum-garnet) laser beams that are widely used for metal welding, and therefore will require a high-energy laser beam. Additionally, with laser welding of aluminum, aluminum alloy, copper, or copper alloy, there exist the problems that the weldability varies greatly depending on influences of surface conditions, and that, as with laser welding of other materials, the occurrence of spatter is unavoidable.
With ultrasonic welding, a large energy is required because of the high thermal conductivity of aluminum, aluminum alloy, copper, and copper alloy, which are the materials to be welded, and there is also the issue that the positive electrode active material and negative electrode active material are prone to fall out due to ultrasonic vibration during the welding.
With resistance welding, furthermore, there are the issues that a large current has to be input in a short time because of the low electrical resistance and high thermal conductivity of aluminum, aluminum alloy, copper, and copper alloy, which are the materials to be welded; that there is risk of the resistance welding electrode rods becoming fusion-welded to the collector members during the resistance welding, and that melting or sparks can occur in places other than the weld portions.
The three welding methods each have their merits and drawbacks as described above. However, in the interest of productivity and economy, it is preferable to employ resistance welding, which has long been widely used as a method for welding between metals. However, the electrode assemblies in the lithium ion secondary batteries or other prismatic sealed secondary batteries used in EVs and HEVs have a structure in which positive electrode plates and negative electrode plates are stacked or wound with separators interposed therebetween. Furthermore, the substrate exposed portions of the positive electrode plates and negative electrode plates are disposed so as to be located on differing sides to each other, with the stacked positive electrode plate substrate exposed portions being welded to the positive electrode collector member, and likewise with the stacked negative electrode plate substrate exposed portions being welded to the negative electrode collector member. Where the capacity of a lithium ion secondary battery or other prismatic sealed secondary battery used for an EV or HEV is large, the number of these stacked positive electrode plate substrate exposed portions and negative electrode plate substrate exposed portions will be extremely large.
JP-A-2003-249423 discloses the invention of a storage element having an electrode assembly formed of positive electrode plates and negative electrode plates wound into a flattened shape with separators interposed therebetween, in which the substrate exposed portions of each electrode are divided into two bundles for welding to the collector member, in order to render small the stacking width of the respective electrode substrate exposed portions that project out from the separators. The structure of the storage element disclosed in JP-A-2003-249423 will now be described using FIGS. 9 and 10. FIG. 9A is a cross-sectional view of an electrical double layer capacitor that serves as the storage element disclosed in JP-A-2003-249423, FIG. 9B is a cross-sectional view along line IXB-IXB in FIG. 9A, FIG. 9C is a cross-sectional view along line IXC-IXC in FIG. 9A, and FIG. 10 is a view showing the welding process between the electrode substrate exposed portions and collector member in FIGS. 9A to 9C.
As FIGS. 9A to 9C show, the storage element 50 has a wound electrode assembly 51 in which positive electrode plates, negative electrode plates and interposed separators (all of which are not shown in the figures) are stacked and wound in a flattened shape, and this wound electrode assembly 51 is disposed inside a prismatic outer can 52 made of aluminum. The positive electrode collector member 53a and negative electrode collector member 53b of the storage element 50 have a U-shaped wing portion 54a or 54b, respectively, formed at one end and connected to the substrate exposed portions 55a of the positive electrode plates or the substrate exposed portions 55b of the negative electrode plates, respectively, with the other end being connected to the positive electrode terminal 56a or negative electrode terminal 56b, respectively. Furthermore, the substrate exposed portions 55a of the positive electrode plates are divided into two bundles, of which one is welded to one outer side face of the U-shaped wing portion 54a and the other to the other outer side face, and likewise, the substrate exposed portions 55b of the negative electrode plates are divided into two bundles, one of which is welded to one outer side face of the other U-shaped wing portion 54b and the other to the other outer side face.
For the positive electrode, for example, ultrasonic welding is performed as follows, as shown in FIG. 10. One of the two split bundles of substrate exposed portions 55a of the positive electrode plates is disposed on an outer face of the U-shaped wing portion 54a, a horn 57 of an ultrasonic welding device (not shown in the figure) is brought into contact with the outer surface of the substrate exposed portions 55a, and an anvil 58 is disposed on the inner surface of the U-shaped wing portion 54a. Note that the other bundle of the substrate exposed portions 55a of the positive electrode plates is ultrasonically welded with the same method, and likewise with the negative electrode.
In the case where the two split bundles of positive electrode plates or negative electrode plates are resistance welded, one will consider either the method of welding each bundle separately or the method of series spot welding the bundles simultaneously. Of these, the series spot welding method will be preferable in view of the smaller number of weldings. With the long-used series spot welding technique, in the case where, as shown in FIG. 11, members to be welded 73 and 74 are welded at two spots coaxially with a pair of resistance welding electrode rods 71 and 72, the method that has mainly been used is to interpose a U-shaped welding piece 75 in the intermediate space and perform the weldings at the top and bottom of the U-shaped welding piece 75. This method is in wide general use because the U-shaped welding piece 75 can be fabricated with ease from flat sheet metal and because it is easy to fabricate projections that will render the resistance welding both easy and stable.
The invention disclosed in JP-A-2003-249423 yields the advantage that a width of the positive electrode exposed portions and of the negative electrode exposed portions can be rendered small, and therefore volumetric efficiency of the storage device will be good. However, with this invention, there exist problematic aspects that will render the manufacturing equipment complex. These include the fact that several weldings are required in order to weld the positive electrode plates and the negative electrode plates to the positive electrode collector member and negative electrode collector member, respectively; and furthermore, that an open space is needed in the central portion of the wound electrode assembly in order for disposition of the welding-purpose U-shaped wing portions of the positive electrode collector member and negative electrode collector member, and that it is necessary to dispose an anvil in the interior of the U-shaped wing portions during the ultrasonic welding.
In addition, although it is stated in JP-A-2003-249423 that the ultrasonic welding method will preferably be used for the process of welding the electrode plates, a number of winding turns in the embodiments is 16 (8 for each of the two split bundles), and a stack thickness is 320 μm. As opposed to this, in large-capacity sealed batteries such as the lithium ion secondary batteries for EVs and HEVs, the number of stacked positive electrode substrate exposed portions and negative electrode substrate exposed portions is much greater than in the case of the invention disclosed in JP-A-2003-249423, and moreover the stack thickness is far larger.
Therefore, with large-capacity prismatic sealed batteries such as the lithium ion secondary batteries for EVs and HEVs, in order to use the ultrasonic welding method to weld in a stable condition the stacked positive electrode substrate exposed portions and negative electrode substrate exposed portions to the collector members, a large application of pressure is required to fit the stacked positive electrode substrate exposed portions and negative electrode substrate exposed portions tightly against their respective collector members, and a large energy is required to make the ultrasonic vibration reach as far as the other ends of the stacked positive electrode substrate exposed portions and negative electrode substrate exposed portions. With the invention disclosed in JP-A-2003-249423, the pressure application and ultrasonic energy have to be sustained by the anvil disposed in the interior of the U-shaped collector members, which means that the anvil must have considerable rigidity, and in addition it is extremely difficult in technical terms to find stable welding conditions under which an anvil of the size that can be provided in the collector member interior will sustain the large pressure application.
Furthermore, with the long-used method shown in FIG. 11, the positive electrode substrate exposed portions and negative electrode substrate exposed portions can each be series-welded with a single welding, but measures such as providing a pressure receiving piece 76 in the interior of the U-shaped welding piece 75 and/or a metal block for power conduction are needed in order to eliminate distortion of the U-shaped welding piece 75 due to pressure application by the welding electrode rods 71 and 72, and such complexification of the welding equipment has been an issue.
In JP-UM-A-58-113268 there is disclosed an electrode plate substrate yoke 80, shown in FIG. 12, in which electrode substrate groups 84a and 84b, formed by splitting into two bundled groups the substrates 84 of an electrode assembly 83, are placed against the side faces of the base portion 82 of a collector member 81 and integrally series spot-welded thereto together with a pair of stiffening plates 85a and 85b disposed on the outer sides of the electrode substrate groups 84a and 84b. 
JP-A-2000-40501 discloses a flat wound electrode battery 90 that, as shown in FIGS. 13A and 13B, has a flattened wound electrode assembly 93 with positive electrode plates and negative electrode plates wound in such a manner, with separators interposed therebetween, that positive electrode substrate exposed portions 91 and negative electrode substrate exposed portions 92 are disposed on opposing sides; and in which, using for example a positive electrode terminal 94 consisting of a rectangular connecting part 94a that has edge portions made into curved surfaces and that fits into the central hollow space 91a around which the positive electrode substrate exposed portions 91 are wound, a terminal part 94b that projects longitudinally in the flattening direction, orthogonal to the winding axis direction, and a short connecting part 94c that connects such two parts, electrical connection is effected by fitting the terminal part 94b of the positive electrode terminal 94 into the central hollow space 91a around which the positive electrode substrate exposed portions 91 are wound (see FIG. 13A), then performing series spot welding on both sides of the positive electrode substrate exposed portions 91.
However, with the series spot welding methods disclosed in JP-UM-A-58-113268 and JP-A-2000-40501, the substrate exposed portions of the positive electrode plates and negative electrode plates are split into two groups and series spot welded directly to the two sides of the positive electrode terminal or negative electrode terminal, respectively, and because such welding surfaces on the positive electrode terminal or negative electrode terminal are flat surfaces, it has been difficult to render high the strength of the weldings between the positive electrode terminal or negative electrode terminal and the substrate exposed portions of the positive electrode plates or negative electrode plates, respectively, and to render small the variation in the internal resistance of the welds. In addition, there has been the issue that the positive electrode terminal and negative electrode terminal must be substantial bodies, which means that the mass of the positive electrode terminal and negative electrode terminal will be large.
In large-capacity prismatic sealed secondary batteries such as the lithium ion secondary batteries for EVs and HEVs, the number of stacked positive electrode substrate exposed portions and negative electrode substrate exposed portions is extremely large, and moreover aluminum or aluminum alloy is used for the positive electrode substrates and positive electrode collector, and copper or copper alloy for the negative electrode substrates and negative electrode collector. Since aluminum, aluminum alloy, copper and copper alloy are materials with low electrical resistance and with good thermal conductivity, it is difficult to render high the strength of the welding between the positive electrode substrate exposed portions and positive electrode terminal and between the negative electrode substrate exposed portions and negative electrode terminal, and still more difficult to render small variation in the internal resistance of the welds.